This invention relates to a method and apparatus for controlling an inverter for driving an AC motor, and more particularly to an inverter of a pulse width modulation system (PWM) for variable speed control of an induction motor.
In recent days, in small electronic appliances of the home-use type using AC motors, particularly an air conditioner which uses an induction motor for a compressor motor, there has been used an inverter of the PWM system. In this inverter, an induction motor is variably speed driven by varying the voltage-frequency characteristics of the AC power supplied to the induction motor in accordance with a variation of load. Such use of the inverter saves power consumption and realizes effective device operation.
The inverter of this type is arranged such that an appropriate number of semiconductor switching elements are connected in a bridge fashion. Each semiconductor switching element responds to a PWM control signal supplied to its control electrode and executes the DC power to AC power conversion. The converted AC power is supplied to the induction motor as a load. Since the AC power contains the voltage-frequency component dependent on the PWM control signal, the induction motor is variably speed driven.
In case of an air conditioner, for example, the PWM control signal is generated according to a frequency control command signal which is based on set temperature data and actual detected temperature data. Various types of generating systems for generating the PWM control signal have been known. The PWM control signal generating system of the prior art largely depends on the low power loss characteristic in an inverter system for variably driving the AC motor, the starting characteristic of the motor, the noise characteristic, and the like. However, none of the conventional inverter control systems have been designed on the basis of detailed and exact analysis of performance factors and correlation among these factors of the PWM control signal generating systems. Therefore, the conventional systems have many disadvantages to be solved, although, of course, there are some advantages.
In this respect, there has been a strong demand for new inverter control systems free from the above problems, in the industrial fields using various types of inverter systems.